Discharge of biceps brachii motor units is modulated by load compliance and forearm posture.

The purpose of this study was to compare the discharge characteristics of motor units in the biceps brachii during brief isometric contractions of the elbow flexors as subjects matched either a target force or a target joint angle with the forearm placed in one of two postures. One task required force control to exert a constant force against a rigid restraint (force task), whereas the other task involved position control to maintain a constant elbow angle while supporting an inertial load (position task). The left arm of right-handed subjects was rotated forward so that the upper arm was horizontal and the forearm was vertical. When the elbow flexor muscles were contracted, the wrist exerted a force in a horizontal direction. Subcutaneous electrodes were used to record the discharge of 20 motor units in neutral and supinated forearm postures during both force and position control. Motor unit recruitment thresholds ranged from 1.3 to 37.9% of maximal voluntary contraction force. Discharge rate was similar at the start of the force and position tasks in both the neutral posture (13.1 +/- 0.6 and 12.6 +/- 0.6 pps, P = 0.54) and the supinated posture (14.7 +/- 1.6 and 14.0 +/- 0.9 pps, P = 0.4) and declined during both tasks in the two forearm postures (P < 0.001). Nonetheless, the decrease in discharge rate (P < 0.001), increase in the coefficient of variation for interspike interval (P = 0.04), and increases in the standard deviation of acceleration (P = 0.02) were greatest for the position task in the supinated posture. These findings indicate that the influence of load compliance on the adjustments in motor unit activity during brief isometric contractions with the elbow flexors was modulated by changes in forearm posture.